A Programmable Logic Controller (“PLC”) is a digital computer typically used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures, which will be referred to generically herein as “industrial equipment.” Unlike general-purpose computers, a PLC is designed for multiple inputs and output arrangements (sometimes referred to as “channels”), extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. A PLC is an example of a real-time system, since output results must be produced in response to input conditions within a limited time for proper operation of the machinery.
A PLC can be used to monitor switches, such as power on/off switches, emergency cut-off switches, etc. of industrial equipment. The voltages handled by such switches tends to be relatively high, e.g. 24-240 volts. Also, the voltages handled by the switches can be direct current (DC) or alternating current (AC). However, the electronic components of the PLC typically operate at much lower DC voltages, e.g. 3.3-5 volts. Therefore, it is important to provide galvanic isolation between high voltage and low voltage sections of a PLC to prevent damage to, and possibly unsafe operation of, the PLC.
In PLCs input detection circuits (“digital inputs”) have been used to convert AC or DC high voltage inputs into low voltage DC outputs. Digital inputs are so-named because they are typically used to detect when a high-voltage switch is opened or closed (a type of binary action), although the actual input voltage is often at least partially analog in nature (e.g. AC, frequency components, transients, etc.). Historically, digital inputs included relays and were referred to as “relay logic.”
Galvanic isolation is a principal of isolating functional sections of electrical system to prevent current flow, e.g. no metallic conduction paths are typically permitted between the functional sections. However, energy or information can be exchanged between the functional sections by other methods, such as by capacitive, inductive, optical, acoustic or mechanical coupling
Optical isolators have been used in digital inputs to provide galvanic isolation between the high voltage and low voltage circuitries. For example, a pulse width modulator (PWM) can be coupled to the high voltage input signal from a high voltage switch to drive the input of an optical isolator. A low voltage pulse width demodulator can recover the input signal from the output of the optical isolator. However, such a circuit requires a power supply (e.g. 20-50 mW) to operate the PWM which adds expense to input circuitry. Furthermore, since PLCs may have many input channels, e.g. 10, 20 or 40 input channels, a like number of power supplies (often including transformers) must be provided if galvanic isolation is maintained for each channel.
The power supplies required for the PWM tend to be expensive, cumbersome, space-consuming and increase the power consumption of the circuitry. For this reason, some PLC's group their inputs so that they can share a power supply, but this will not provide galvanic isolation within the group.
In certain applications where the input voltage is known a simple resistive divider can be used to power an optical isolator of a digital input. This is advantageous in that galvanic isolation can be provided very inexpensively. However, simple digital inputs of this type will only work at known voltage (and therefore current) levels, and therefore cannot be used as a “universal” digital input for a significant range of input voltages.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.